Creating a demonstration liquid mirror telescope

Benjamin Coe, Oneonta Senior High School; Marty Cohen and John Noé, Laser Teaching Center, Stony Brook University

It is a familiar fact that the surface of a rotating liquid assumes a concave shape. Three hundred years ago Sir Isaac Newton proved that this shape is in fact a paraboloid of revolution described by the equation y = (ω2/ 2g) x2, where ω is the angular velocity in radians per second and g is the acceleration of gravity. Ernesto Capocci first proposed using such a parabolic surface as the primary mirror of a telescope around 1850, but it was only about two decades ago that the numerous technical challenges were overcome and a successful full-scale telescope created. Liquid mirror telescopes are much cheaper than comparable glass mirror telescopes. They can only observe objects directly overhead, but in practice this isn't an important limitation.

The goal of our project is to build and test a basic liquid mirror telescope. Our telescope is simply a shallow container of water about 30 cm in diameter placed on a record player. The rotational speed of the record player can be varied between 30 – 36 rpm or 40 – 46 rpm. An effective container must first and foremost be watertight. It also has to be rigid, so as to not bend under stress from the water, and balanced to continue spinning well. Finally, there must be a way to keep the container centered on the spinning platform. Our current container is made from two plastic gardening pots, which are about 15 cm and 33 cm in diameter, respectively. A hole that provides a snug fit around the spindle of the record player was drilled in the exact center of the smaller pot. An indent on the bottom of the larger pot keeps it centered on the smaller one, and hence concentric with the axis of the turntable.

Currently two related types of measurements are being carried out with our prototype device and a precision 0.5 meter height gauge. The first procedure creates a map of the parabolic surface by probing its depth at several locations. The second determines the focal length of the mirror by projecting an image of a small incandescent lamp filament on the ceiling. Both results can be compared to predictions derived from the formula above. Once these measurements are completed we will place a CCD camera near the focal point and use the resulting telescope to create an image of a simulated star.

We would like to thank the Simons Foundation for funding this research, Jeff Slechta for his expert assistance, and Prof. Harold Metcalf for establishing and supporting the Laser Teaching Center.